Four small bombs exploded in Colombia in what police said may have been a protest by leftist guerrillas against the presence of U.S. President Barack Obama in the country for the Summit of the Americas.

A senior police source in the capital Bogota told Reuters that two explosives were placed in a ditch in a residential area near the attorney general's office and the U.S. Embassy.

"There are windows broken, but nobody hurt or killed," the source added.